Aspergillus niger secretes an alpha-glucosidase (alpha-glu) into its growth medium. This enzyme also exists in a membrane-bound form which may serve as an intermediate in the secretory process. To study the movement of this alpha-glu through cellular membranes and the possible differences between it and the secreted form, membrane fractions will be isolated and subjected to SDS-PAGE slab gel electrophoresis followed by transfer of the resolved proteins to nitrocellulose. Various forms of alpha-glu will be detected by the application of 125I-anti-alpha-glu IgG onto the nitrocellulose and autoradiography. Similar treatment of membrane fragments from cells labeled with (35S) methionine for various periods of time will allow a determination of the time course of membrane flow. Cell-free translation of A. niger mRNA in the presence or absence of membranes and analysis of the radiolabelled products by SDS-PAGE after indirect immunoprecipitation will demonstrate the nature of the initially produced alpha-glu and its mechanism of insertion into the membrane.